[...]
Why? First, it is worth observing that the very nature of OBS makes it
difficult to pursue that goal. Because in OBS, for any project/package
there is only one spec file - and therefore package version - for ALL
repositories.

In other words, it would be impossible for a maintainer to configure a
package to build foobar-1.2 on openSUSE 11.3 but foobar-2.1 on
openSUSE 12.1.

First and foremost get the package into the distro instead of
building it in some random repo. The whole point of the distro is to
be a collection of packages that work together.

Agreed.

Secondly, you can have different versions of foobar in the same
project by naming the source directories e.g. foobar and a
foobar.openSUSE_xy. Only the latter would be build enabled for
openSUSE xy while the former builds for everything else.

That works for one project but is impractical as a general solution
... it would require an explicit action (and new source directory) for
every package in every OBS project every 8 months.